The evolutionary history of the Juglandaceae has been well reviewed by Manchester (1987). The family is first distinguishable in the fossil record as distinctive pollen grains which occur from the late Cretaceous, about 135 million years ago (Frederiksen and Christopher, 1978). The subtribe Carinae (referred to by Manchester as the tribe Hicoreae) is believed to have evolved from these primitive ancestors about 70 million years ago, via intermediate species which can be found from the late Cretaceous to the late Paleocene (Nichols and Ott 1978). Primitive members of the Carinae were well distributed across North America and Eurasia during the Eocene, which began about 50 million years ago. The first recognizable Carya fruits occur in strata of the lower Oligocene, about 34 million years old, found in Colorado (MacGinitie 1953) and in Germany (Gregor 1978). Carya fruits are found in China in Miocene strata, about 20 million years old (Hu & Chaney 1940). The genus became extinct in Europe in the Pleistocene about 2 million years ago, possibly as a result of glaciation. In the same time period, the range of the genus in North America was greatly reduced and species in western North America became extinct. Fossils believed to represent living species are found in North America from the Pleistocene (Lamotte 1952).
Manchester (1987) noted that during the Paleocene and Eocene, the greatest number of genera of Juglandaceae occur in North America, indicating that this area is the likely place of origin for the family. Migration occurred across Beringia, as indicated by the presence of fossil Carya in Siberia. This explanation is consistent with the timing of the occurrence of Carya in Asia.
The earliest fossil fruits of Carya, found in Oligocene strata in North America and Europe and in Miocene strata in China, lack secondary septa in the walls of nuts (Manchester 1987). Manchester (1987) suggested that the development of secondary septa and increased shell thickness were evolutionary responses to predation by rodents, and offered as evidence the synchronous evolution of squirrels and complex seed packaging modifications in hickories (both occurring after the Oligocene). An exception to that trend of evolutionary development is pecan, which lacks secondary septa and has a thin shell, possibly due to selection by man.
The other observable trend in the evolutionary history of the genus is toward increasing pollen size (Manchester 1987), a feature which has been reported among extant species to be correlated with polyploidy (Stone 1963, Whitehead 1963). It is possible that the extant "diploids" (n=16) in the genus are the result of increased ploidal level from ancestors having only 8 chromosomes. Increased ploidal level could account for the reported increased pollen grain size which occurred at the boundary between Wilcox and Claiborne (early Eocene) formations in the Mississippi Embayment (Tschudy 1973) and between Sabinian (late Paleocene) and Jacksonian (late Eocene) formations in South Carolina (Frederiksen and Christopher 1978).
Manchester (1987) noted that the Juglandaceae underwent a phase of rapid evolution during the Paleocene, establishing most morphological patterns which characterize modern tribes and genera within about 10 million years. Evolutionary rate slowed from the middle Eocene onward, with one example of post Eocene evolution being the development of the secondary septum and internal locule ridges in Carya.
Information on the ancient distribution of plant populations is often based on studies of pollen recovered from strata that increase in age with increasing depth. Hickory pollen can be distinguished from that of other genera, being characteristically triporate, paraisopolar (the three pores are never exactly equatorial in position but are drawn toward the distal pole) and suboblate, with a textate surface (Whitehead 1965). Pollen diameter averages 46 um and ranges in diameter from 38 to 55 um in different species, with diploid species having the smallest pollen grains (Stone 1963). Despite that pattern of difference, species of Carya can not be reliably separated from each other on the basis of pollen characteristics. The distribution of hickory in North America during the past 20,000 years has been mapped based on pollen records (Delcourt and Delcourt, 1987). Those records allow generalizations concerning the movements of populations into geographic regions, as well as demographic changes in dominance structure of the entire forest population. Hickory advanced rapidly northward from a limit of 34o N, beginning about 16,000 yr Before Present (BP). It reached its current northern limit of 45 o N by 8000 yr BP, with rates of advance as high as 354 m/yr. The period of fastest northward advance for Carya was between 14,000 and 12,000 yr BP. Hickory populations decreased from a mean of 9% dominance in the full-glacial interval (20,000 to 16,500 yr BP) to 7% for the late-glacial (16,500 to 12,500 yr BP) and early Holocene (12,500 to 9,000 yr BP) intervals, then remained between 5% and 6% for the mid- to late Holocene (from 9,000 to 6,000, and 6,000 to 500 yr BP, repectively). As hickory population mean dominance values declined over that period, it's area and maximum dominance values increased in a pattern that indicates a "K-migration" strategy. Such plants are typically long-lived, late-successional taxa that tend to colonize and then successfully maintain populations in nutrient-rich soils. K-strategists tend to be shade-tolerant, and invest more energy establishing biomass than in producing seed. Propagules are fewer but larger, remain viable longer in the soil, and are dispersed by gravity and animal vectors (Delcourt and Delcourt, 1987).
LJ Grauke , Research Horticulturist & Curator
USDA-ARS Pecan Genetics
10200 FM 50
Somerville, TX 77879
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